A semipermeable membrane, with which separation of CO2 is possible, is described in WO 2005/089907 A1 and the publication “Novel Fixed-Site-Carrier Polyvinylamine Membrane for Carbon Dioxide Capture,” T.-J. Kim, B. Li, M. B. Hägg, Journal of Polymer Science; Part B: Polymer Physics, Vol. 42, 4326-4336 (2004).
The separation of CO2 from carbon dioxide-containing gas mixtures, which contain halogenated hydrocarbons, as they occur above all in the field of medicine, in which artificial respiration is combined with the administration of gaseous anesthetics, is a technically demanding task.
It has been known for a long time in various respiration processes that the breathing gases administered are prepared in a cyclic process for reasons of economy, safety and environmental protection, and components being consumed are replaced by supplementary feed and components whose concentration increases are maintained below a critical concentration value by separation.
The most important component whose concentration increases in a respiration system is carbon dioxide. An increase in the carbon dioxide concentration in respiration systems, as a result of which concentration values of half of one percent would be exceeded, is generally to be avoided. The breathing in of carbon dioxide at concentrations of 0.5% may already cause headache and increase the respiratory stimulus, and it displaces other gases that are necessary for the supply of a patient.
The percentage of carbon dioxide that is to be removed depends on the particular application. Expiratory breathing air contains approximately 5% carbon dioxide. This concentration is to be reduced in cyclic processes to a maximum of 0.5%. In closed breathing circuits, as they occur in space applications or in mining, the percentage of carbon dioxide is usually to be reduced to markedly lower concentrations. The gas mixture from which the carbon dioxide is to be removed is likewise subject to variations in terms of its composition from one application to the next.
In medical applications, the breathing gas mixture frequently contains nitrous oxide, xenon, helium as well as volatile anesthetics and a number of trace gases that are present because of physiological reasons, such as methane or acetone, besides the components of the air, namely, nitrogen, oxygen, argon and water. The volatile gaseous anesthetics are usually halogenated hydrocarbons.
It is known that carbon dioxide can be bound to adsorbing substances in cyclic processes. These substances are based, as a rule, on hydroxides, for example, calcium hydroxide, sodium hydroxide and potassium hydroxide or mixtures of these components, which are frequently called “breathing lime.” These components have a number of drawbacks. Soda lime is not reusable. Lime cartridges packaged in the usual manner therefore have a limited capacity and are to be replaced cyclically as a function of this capacity. Soda lime is corrosive in a moist environment, and special measures must therefore be taken for effective protection against the dust. The effectiveness of absorption declines when the lime dries out. The state of saturation of a lime filling cannot be displayed reliably with sufficient certainty. Moreover, lime cartridges offer a considerable and possibly variable flow resistance to the breathing gas flow, which may lead to a marked loss of comfort in cyclic systems with lime-based absorption. The absorption reaction leads to a considerable release of heat of reaction under special conditions, which may bring about a cracking reaction in the presence of halogenated hydrocarbons, as a result of which toxic fractions of these substances may possibly be released.
Furthermore, it is known that the separation of carbon dioxide from a gas mixture can be carried out by guiding the gas mixture past a semipermeable membrane (DE 21 40 902). However, prior-art semipermeable membranes have insufficient selectivity in mixtures that contain carbon dioxide and nitrogen, carbon dioxide and nitrous oxide or carbon dioxide and halogenated hydrocarbons, or they are characterized by variable selectivity, which may be affected by swelling effects or a strong moisture dependence, so that use for patient- and safety-relevant applications is not considered. Another drawback of conventional semipermeable membranes is their lack of stability against halogenated hydrocarbons, which makes them unsuitable for long-term applications, in which permanent or varying exposure to halogenated hydrocarbons is to be expected.
Other prior-art processes, which are based on the use of liquid sorbents such as primary, secondary or tertiary amines in a solution, for example, methanolamine or mixtures thereof, are not considered for application in respiration systems because the processes are either technically or chemically too complicated and hence expensive or they are to be ruled out because of an excessively high vapor pressure of the sorbent, because they could thus give rise to toxic concentrations in the area in which the breathing gas is guided.